This invention relates to the preparation of a new cell strain, to the cultivation of viruses therein, and to the production of vaccines from said viruses.
There are a number of problems associated with the growing of viruses for the production of vaccines. Viruses are grown on cell cultures and a suitable cell culture must be found for each specific virus from which a vaccine is to be made. A specific virus is generally found to grow or may be adapted to grow in only one or a few cell cultures. To be economically feasible in commercial vaccine production, the rate of growth of a virus in a particular cell culture must be reasonably rapid. In addition, the rate of growth of the cells in a cell culture must also be sufficiently rapid in order to provide reasonable quantities of cells on which the virus can grow.
Cell cultures generally used for virus cultivation are either primary cell cultures or they are cell strains or cell lines. Primary cell cultures are started from cells, tissues or organs taken directly from an organism and is regarded as a primary cell culture until it is subcultured for the first time. A cell line or cell strain arises from a primary culture at the time of the first subculture. They are either "finite" or "continuous", i.e., they are "finite" if the cells will grow for a limited number of passages before senescence occurs or they are "continuous" if they appear to be capable of growing for an infinite number of passages.
Primary cultures are generally undesirable in commercial vaccine production since fresh tissue must be obtained from new donors for every lot of vaccine produced, with the attendant risk of viral, bacterial and fungal contamination with each new batch of tissue.
Although there are several continuous cell lines available for veterinary vaccine manufacture, such cell cultures are not considered acceptable in the production of vaccines for human use. The reason for this is that when a cell line becomes continuous, the cells are no longer diploid but have become heteroploid. It is generally believed that such heteroploid cells may carry a greater potential for oncogenicity. The most desirable cells for cultivation of viruses to be used in vaccine production, particularly for human use, are therefore those from finite cells strains or lines which remain diploid and have good growth rates which are substantially uniform over a large number of passages. Oftentimes, attempts to obtain such strains result in those which show senescence after only a few passages or the cells may become heteroploid even though the strain is a finite strain.
At the present time, vaccines for the treatment or prevention of rabies have several shortcomings, particularly in the treatment of humans. One such vaccine derived from rabies virus grown on duck embryos is of low antigenicity and thus requires a prolonged and painful course of treatment. Another cell culture for rabies virus cultivation which is suitable for a vaccine for human use is a finite diploid cell strain derived from human fetal lung tissue. There are some who feel diploid cell strains other than those derived from human tissues would be more desirable for virus cultivation intended for human vaccines since the possibility for other cell strains to harbor a latent or undetectable virus to which humans might be susceptible would be more remote.
A fibroblastic cell culture derived from fetal dog lung tissue was described by de Ratuld and Werner (Ann Inst. Pasteur 112, 802, 1967) which they sub-cultured for between 10 and 30 passages in order to determine whether this particular organ (lung) in the dog fetus carried a Herpes-like virus. These same authors in U.S. Pat. No. 3,462,526 described the use of a fibroblastic cellular strain from fetal dog lung for the cultivation of this same virus.